Implications of the embedded through-section technique for improving the resilience and sustainability of existing reinforced concrete infrastructure

The strength enhancement of structurally deficient concrete infrastructure is an application of considerable economic and strategic importance, particularly in the case of bridges. In the United Kingdom alone, it has been estimated that there are approximately 10,000 bridges on the strategic road network and 150,000 bridges on local roads, of which a considerable number need strengthening or replacement. The estimated cost of assessing and strengthening such structures is in excess of £4 billion. Other countries, e.g. the United States, are faced with the same challenge, so emphasising the global significance of the issue.

During the past two decades, fibre reinforced polymer (FRP) reinforcement has gained acceptance as strengthening systems for existing reinforced concrete (RC) structures. The use of FRP strengthening systems is advantageous due to their excellent mechanical and durability properties. Extensive research has resulted in approved FRP flexural strengthening methods for RC structures. In contrast, FRP shear strengthening of RC structures is not yet fully understood.

To date, FRP shear strengthening systems for existing RC structures have primarily been applied as externally bonded (EB) or near-surface mounted (NSM) reinforcement. In order to utilise these systems, both sides of individual beam webs must be accessible. However, it is difficult to provide such an access in several practical situations. Moreover, laborious and time-consuming surface or groove preparation is required to ensure adequate bond between the concrete and the EB or NSM systems respectively. Furthermore, unless proper anchorage is provided, both the EB and NSM systems debond from the concrete at a stress level of 20% to 30% of the ultimate strength of the FRP reinforcement.

The embedded through-section (ETS) technique is a recently developed shear strengthening method for existing RC structures. In this method, vertical holes are drilled upwards from the soffit in the shear spans of existing RC beams. High viscosity epoxy resin is then injected into the drilled holes and FRP bars are embedded into place. The ETS technique provides higher strengthening effectiveness than that provided by the EB or NSM systems. Other advantages of the ETS technique include higher protection against fire and vandalism, less epoxy consumption, and no need for access to the top slab or time-consuming surface preparation.

Research investigating the shear behaviour of RC beams strengthened with ETS FRP bars has been limited. All beams tested to date had effective depths of less than 400 mm. This is unrepresentative of several practical situations where RC bridge beams have significantly higher effective depths. Moreover, the effect of other parameters that influence the structural behaviour of the strengthened beams, such as the shear span to effective depth (a/d) ratio and FRP bar type, has not been sufficiently investigated. A proper understanding of the effect of the above-mentioned parameters on the strengthened behaviour is vital for the best utilisation of the ETS technique.

This project will investigate, experimentally and numerically, the effect of a/d and FRP bar type on the behaviour of realistically sized RC beams strengthened in shear with ETS FRP bars. The combination of experiments and numerical techniques will ensure an integrated modelling approach that will inevitably lead to a better understanding of the strengthened behaviour. The experimental results will be used to check the accuracy of current design standards and improve their predictions where needed. The insight gained from this project will enable the utilisation of FRP reinforcement for improving the sustainability and resilience of existing RC infrastructure. The concepts encompassed in this work will underpin our understanding of the behaviour of FRP-strengthened concrete structures and thus have parallel implications in a variety of other areas of concrete construction.

The primary output of this research will be a better understanding of the behaviour of concrete structures strengthened in shear with embedded through-section (ETS) fibre reinforced polymer (FRP) reinforcement. This will enable the utilisation of such a promising technique for improving the sustainability and resilience of existing concrete infrastructure.

The proposal, through the steering group, brings together stakeholders from a broad range of disciplines (The Highways Agency, academia, and industry). The proposal's deliverables will aid policy makers, planners and other high-level decision makers in ensuring more resilient and sustainable concrete infrastructure exist; and ultimately benefit the societies that they serve, the UK economy, and international research agendas. The beneficiaries of the project include:

1. The strengthening and repair sector of the UK Civil Engineering industry: this project will provide consultants, designers, and contractors with new knowledge and methods that will give them a market advantage in international terms.

2. The international academic community, which will benefit from both the experimental and numerical outputs of this research as they will provide insight into the behaviour of concrete structures strengthened with ETS FRP reinforcement.

3. FRP manufacturers who have already developed interest in the ETS technique (see for example the statement of support from Fyfe Europe) as it is generally more practical than other shear strengthening methods.

4. The Post-doctoral Research Associate and their future employer, who will gain invaluable experience from research in this growing field.

5. Infrastructure owners and/or managers, who will be able to make direct use of the research findings to deliver more resilient and sustainable infrastructure, not to mention the long-term environmental benefits resulting from reducing the carbon emissions associated with building new concrete infrastructure.

Finally, this research will also benefit a sector of the public not traditionally targeted by University research, i.e., school children. This will be achieved through four visits to both primary and secondary schools within the West Midlands and through a poster campaign which will be developed from a competition involving such schools. It is envisaged that working closely with such schools will encourage pupils to consider pursing scientific careers and take an interest in challenges facing the UK transport infrastructure. In addition to the school visits and competition, invited lectures and public seminars will be organised to highlight the research innovation, and to present the findings of the project.